Mass Spectrometry & Spectroscopy
Hyphenated electrochemical-Raman spectroscopy: Another dimension for your research
Dr Thomas Touzalin, Metrohm Autolab,
info@metrohm.com
The combination of electrochemical and spectroscopic techniques yields complementary information when studying (electro)chemical reactions. Electrochemical techniques offer precise quantitation with the possibility to analyse solutions at the low part per million (ppm, mg/L) concentration range, or surface processes involving sub-monolayer coverages. The drawback of electrochemical methods is that they offer limited specifi city for the target reaction. The information is rather one-dimensional as researchers can monitor the fl ow of electrons at a given potential, but it is diffi cult and often impossible to attribute the current signal to a single process. Optical spectroscopic methods like Raman spectroscopy provide molecular information and the possibility to monitor chemical processes as they occur.
This article serves as a primer on the basic principles of Raman spectroscopy and elaborates on the combination of electrochemical techniques with Raman spectroscopy as a means of better understanding electrochemical processes. Examples from the recent literature are provided to illustrate the power of hyphenated EC-Raman.
The fundamentals of Raman spectroscopy
Light can interact with matter in several ways including by absorption, transmission, and scattering. The understanding and controlled observation of these interactions make up the majority of spectroscopic analytical tools, allowing researchers to probe the structures, confi gurations, and properties of molecules.
Raman spectroscopy is a technique based on the phenomenon of light scattering during molecular vibration. Unlike infrared spectroscopy, it is not necessary for the frequency of the incident light to match energy levels in the sample molecule for this interaction to occur. Raman spectroscopy uses a single wavelength (frequency) of light to irradiate the sample. The incident radiation briefl y distorts or polarises the electron cloud around the nuclei into a temporary virtual state. Since the virtual state is unstable, it reverts, and the photon is re-emitted. This process is called light scattering [1]. The polarisation of the electron cloud causes movement of the electrons and also the nuclei. Movement of electrons is relatively «easy» because electrons have little mass; this type of interaction results in a scattered photon that has the same energy as the incident photon. This process is known as elastic or Rayleigh scattering and it is the dominant scattering process. However, the polarisation can also result in nuclear motion when energy is transferred from the incident photon to the molecule, or vice versa, resulting in what is known as inelastic or Raman scattering. Inelastic scattering results in measurable energy shifts; the scattered photon resides at one vibrational unit of energy different from that of the incident light [1,2].
An energy level diagram (also known as a Jablonski diagram), such as the one shown in Figure 1, is useful for illustrating these processes. Notice that there are two types of Raman interactions shown in the diagram: Stokes and Anti-Stokes interactions. In a Stokes Raman interaction, energy is transferred to the molecule and the resulting photon has lower energy (higher wavelength) than the incident light. In Anti-Stokes interactions, the molecule transfers energy to the photon so the resulting photon is of higher energy (shorter wavelength) than the incident light. The Jablonski diagram shows how these situations can occur. The Anti- Stokes interactions start with the molecule residing in an excited vibrational state and ending up in a lower energy state, whereas the Stokes interactions start with the molecule residing in the lowest energy state. The population distribution in these vibrational states is given by a Boltzmann distribution and depends on the thermal energy (temperature) of the system. In general, at room temperature, the lowest energy states will be the most populated, so the relative contribution of Anti-Stokes interactions is low compared to Stokes interactions. In practice, Stokes interactions are the most studied in the Raman spectrum, and often Raman spectra are presented with only positive Raman shift values, corresponding to the Stokes interactions [1,2].
Figure 1. Raman scattering. As previously noted, elastic scattering is the dominant process. Only one in 106
scattering events are inelastic [2]. This is why Raman scattering is said to be
inherently weak. However, there are techniques available to enhance the signal and improve the probability of Raman scattering events. Several of these strategies will be discussed in a later section.
The Raman spectrum explained
A Raman spectrum is a plot of scattered light intensity (in arbitrary units) vs. a property called the Raman shift which is expressed in reciprocal centimetres (cm-1
) - the same
units used in infrared spectra. This can lead to confusion for those who are accustomed to infrared spectroscopy but are new to Raman spectroscopy. The Raman shift is the difference between the measured frequency of the Raman scattered light (measured in wavenumbers and converted to reciprocal centimetres) and the frequency of the incident light source (i.e., the laser). The use of the Raman shift, which is a relative property as opposed to the absolute measured wavelength of the scattered light, differentiates Raman spectroscopy from infrared spectroscopy. Of course, there is a logical reason why Raman spectra are plotted this way, which can be understood by studying the interactions shown in Figure 1.
The Raman shift for a particular transition does not change based on the excitation wavelength, whereas the measured wavelength for the transition will be changed based on the excitation wavelength. This is because the energy of the scattered photons is a combination of the energy of the incident light and the energy transferred during the interaction. However, only the transferred energy is characteristic of the vibrational
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INTERNATIONAL LABMATE - FEBRUARY 2023
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